Field of the Disclosure
The present disclosure relates generally to power converters and, more specifically, to non-isolated Buck converters.
Background
Conventional wall outlets generally deliver a high voltage alternating current (ac) power that needs to be transformed to direct current (dc) power before being used to power many electronic devices. Switched mode power converters are commonly used due to their high efficiency, small size, low weight, and safety protection features, to convert the high voltage ac power to a regulated dc power. For example, switch mode power converters are commonly used in drive circuitry in the lighting industry to provide dc power for various lamp types, such as compact fluorescent lamps (CFLs), Halogen lamps, or light emitting diode (LED) bulbs.
Output regulation in a switch mode power converter is usually provided by sensing a feedback signal from the output of the converter and controlling the power converter in a closed loop. In different control methods, the feedback or control signal may be used to modulate a duty cycle of a switching waveform (referred to as pulse width modulation, PWM), to change a switching frequency (referred to as pulse frequency modulation, PFM), or to disable some of the cycles of the switching waveform generated by the power converter controller (referred to as on-off control). Any of these control methods would result in control of the dc output voltage or current versus the load and line variations.
In applications where the output of the power converter can be accessible by the consumer, isolated converters are typically used to provide galvanic isolation between the output load and the input power line. However, in applications where the output of the power converter is not accessible by the consumer during normal operation, such as lamp drivers for LED (or CFLs or Halogen) bulbs, simple non-isolated converters, such as Buck converters are preferred due to their lower component counts, lower weights, and smaller sizes.
The feedback signal can be directly or indirectly extracted from a sense circuit coupled to the dc output of the converter. In a converter with a regulated output current, such as an LED driver, the current at the output may be monitored/sensed/measured to provide a feedback signal to the controller in order to control switching and the transfer of energy to the converter output. A typical way in prior art to measure the output current is to include a sense resistor at the output of the power converter such that the output current flows through the sense resistor. The resultant voltage across the sense resistor is proportional to the output current. However, the voltage drop across the sense resistor is typically large and often referenced to a voltage level different than that of the power converter controller. Thus, additional circuitry, such as an opto-coupler or a bias winding, is often needed to level shift the voltage across the sense resistor in order to interface with the controller.
The output signal may also be measured indirectly with reference to the controller ground. For instance, in a non-isolated high side switch Buck converter, the output feedback signal may be retrieved indirectly by sensing the current in the inductor, in the freewheeling diode or in the switch with reference to the controller ground.
A known issue in Buck converters is the risk of failure due to a reverse current through the Buck power switch when a negative voltage appears across the switch. This condition could inversely affect operation of power converter, and ultimately result in failure. Specifically, when a controller and a power switch are into a common die or in a single integrated circuit IC package, an inverse current may pass through the substrate, which can result in instability, controller latch, and in some circumstances failure of the power converter.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.